1. Field of the Invention
This invention relates to a hollow rack shaft, and more particularly to a method for manufacturing a hollow rack shaft for a steering system used in an automobile.
2. Description of the Related Art
In the case of a rack-and-pinion type steering system for an automobile frequently used in an automobile industry, the rotation of a steering wheel operated by a driver is transmitted to a pinion. The rotation of this pinion is transmitted to a rack shaft engaged with the pinion and this is converted into a motion in a lateral direction (a longitudinal direction of the rack). Since the rack shaft is connected to the steering rod, the direction of front wheels is changed by a lateral motion of the rack shaft. Since such a steering mechanism as described above is well known in the art, further description will be eliminated.
The aforesaid rack shaft has been attained by gear cutting on a solid, namely not hollow, material. In recent years, as already described in Japanese Laid-Open Patent No. Hei 6-246379, Japanese Laid-Open Patent No. Sho 58-218339 (Japanese Patent Publication No. Hei 4-028582) and Japanese Laid-Open Patent No. Hei 11-180318, fuel efficiency of an automobile has been improved by making a hollow rack shaft to attain the lightweightization of the rack shaft.
A manufacturing method for forming rack is disclosed in Japanese Laid-Open Patent No. Hei 6-246379, which describes that a tube material is inserted into a first forming split die, a primary forming is carried out, then, the aforesaid primary formed material is inserted into a secondary forming split die having, at a part of its inner surface, teeth corresponding to the rack teeth, a semi-circular mandrel is press fitted at one end of it.
A manufacturing method disclosed in Japanese Laid-Open Patent No. Sho 58-218339 describes that a die having teeth corresponding to the rack teeth is pushed to a tube material to form rack teeth under the state that a mandrel is inserted into the tube material surrounded with fixed die.
The manufacturing method disclosed in Japanese Laid-Open Patent No. Hei 11-180318 is an invention invented by Mr. Ohkubo, one of the inventors of the present invention. The manufacturing method comprises the following first to third steps. At the first step, a plate-like material having a substantial rectangular shape as original one is bent into a gutter shape along a center of longitudinal direction. At this time, the central part of the gutter-shaped member is formed into a flat bottom section, and both the side portions are formed into a semi-circular shape having legs therein. At the second step, the rack teeth are plastically formed at the flat bottom portion. At the third step, the gutter-shaped material is bent such that the legs of the central part and the both sides are abutted to each other, and then a hollow rack shaft is formed.
In the case disclosed in the first two gazettes above, the hollow original workpiece is applied as the material, while, in the case disclosed in the third gazette (the gazette of Japanese Laid-Open Patent No. Hei 11-180318), a flat plate workpiece is applied in place of the hollow workpiece, so that the third manufacturing method has some merits in view of material cost, transportation cost and storing cost. Further, according to the third manufacturing method, it is avoidable to give each portion except a tooth bottom portion an excessive thickness in order to maintain the tooth bottom portion at a required wall thickness. Then, the rack shaft can be sufficiently light in its weight.
However, it has become apparent that the manufacturing method disclosed in the gazette of Japanese Laid-Open Patent No. Hei 11-180318 has some problems not considered up to now as described below.
Although, in general, almost the deformation applied to the workpiece by a plastic working operation gives a permanent deformation, temporary deformation, i.e. elastic deformation remains in a part of deformation. As the lead applied by the die is removed, the elastic deformation remaining at each minute element in the workpiece is restored and it is transferred to a new balanced state where each of the forces of the other fine elements keeps its balance. Due to this fact, after the workpiece is removed from the die, the entire workpiece does not take a shape coinciding with the die and its shape deviates from that of the die. In particular, in the case that a degree of forming is widely different in reference to each of the locations at the workpiece, this deviation shows a large value.
FIGS. 1a to 1c show the state when the flat plate workpiece is formed into a gutter-like shape at the first step of the prior art manufacturing method (the final stage of press stroke). In this case, FIG. 1a is a front sectional view for showing a workpiece and a die. FIG. 1b is a sectional view taken along line Bxe2x80x94B of FIG. 1a. FIG. 1c is a sectional view taken along line Cxe2x80x94C of FIG. 1a. The die set at the first step comprises an upper die set 110 including a right upper die 111, a left upper die 112 and a central upper die 113; and a lower die set 120 including a right lower die 121, a left lower die 122 and a central lower die 123. As apparent from these figures, a workpiece W is formed by the lower die set 120 and the upper die set 110 into a gutter-like shape having a flat bottomed central portion 2a, a rack tooth not-yet formed, and having semi-circular bottomed portions 1 extending from the both sides of the flat bottomed central portion 2a. 
Upon completion of the first step, when the upper die set 110 is lifted up to take out the workpiece W, the workpiece W takes a different shape from that of the lower die set 120 and the upper die set 110. FIG. 2 is a sectional view for showing the workpiece W expressed under the state of releasing such a stress as above being exaggerated, as shown in this figure, the workpiece W is warped like a bow with the bottom part of the gutter being placed inside, i.e. the opening of the legs being placed outside.
The reason why such a state occurs is as follows. The original plate workpiece is formed into different shape at each potion, therefore, the degree of deformation given to reach portion differs from others. Namely, the degree of deformation given to the flat bottom portion 2a, the semicircular bottom portion 1 and the interface portion 9 of the workpiece are different from each other. When the load applied to the workpiece is unloaded after plastic working, each portion recovers elastically in correspondingly different manner. Hence the workpiece warps like a bow.
Further, a similar problem may also occur at the second step of the prior art manufacturing method. FIG. 3 is a sectional view for showing the workpiece W and the dies when rack teeth 2b are formed at the portion 2a (FIG. 2) at the second step. The die set at the second step comprises an upper die set 210 including a right upper die 211, a left upper die 212 and a central upper die 213; and a lower die set 220 including a right lower die 221, a left lower die 222 and a central lower die 223. The upper surface of the central lower die 223 is formed with teeth complementary shape to the rack teeth to be formed. In addition, the lower surface of the central upper die 213 is provided with a corrugated shape corresponding to the rack teeth at the surface.
The rack teeth unformed portion 2a of the workpiece W is formed with rack teeth by approaching the central lower die 223 and the central upper die 213 to each other under the state that the semi-circular part 1 of the workpiece W is held between the right upper die 211, the right lower die 221 and between the left upper die 212 and the left lower die 222.
Since many teeth are formed at the rack portion 2b the degree of forming at each location is different. When the workpiece W is removed from the die upon completion of the second step, the workpiece W sometimes shows unintended deformation by the same cause as above. FIG. 4 illustrates one example of the workpiece W in which deformation appeared, in this case, the workpiece W is warped like a bow shape with the bottom part of the gutter being set inside i.e. with the opening part of the leg being set outside.
Even if the third step for butting legs is applied to the workpiece W which has unintended deformation after the second step, this deformation remains, so that as shown in FIG. 5 at its exaggerated form, the workpiece W having unintended deformation, for example, a bow-shaped warp is attained. The workpiece having such a warp as above cannot be used as a rack shaft as it is.
At the second step of the prior art manufacturing method, another problem differing from the aforesaid problem may occur. FIGS. 6a and 6b are a front sectional view and a sectional view taken along line Bxe2x80x94B to illustrate this problem. It should be noted that these figures are illustrated in inverse relation in a vertical orientation against the aforesaid figures. In addition, side clamping dies 203 are auxiliary dies for clamping the workpiece W from side surfaces to prevent the workpiece W from being moved.
As already described above, rack teeth are formed by the central upper die 213 and the central lower die 223. These dies are designed such that the volume of the clearance composed of two dies is slightly larger than the volume of the workpiece W placed in the clearance when the central upper die 213 and the central lower die 223 approach most during forming of rack teeth.
However, in the case that the workpiece W is thicker than the intended thickness or, in the case that a volume of the aforesaid minimum clearance is smaller than the intended volume due to an error in stroke of a press machine, the pressure at this clearance becomes quite high. Due to this fact, a quite high load is applied to the central upper die 213 and the central lower die 223, and in particular, the load is concentrated at either the cavity 5a or 5b. These excessive load or fatigue of die material caused by repeated application of the load may generate a crack 5 (or damage) in dies 213, 223 as shown in FIG. 6a. 
In addition, when the rack teeth are formed, a quite large amount of material must be flown near the teeth ends of the workpiece W, in particular, a crack 5 may easily be generated at either the cavity 5a or 5b. 
Further, at the third step of the prior art manufacturing method, another problem differing from the aforesaid problem is generated. FIGS. 7a to 7c, FIGS. 8a to 8c and FIGS. 9a to 9c are views for illustrating processes in which the hollow rack shaft is formed by bending both legs of the workpiece W so as to be butted to each other. FIG. 7a, FIG. 8a and FIG. 9a are front sectional views. FIG. 7b, FIG. 8b and FIG. 9b are sectional views taken along line Bxe2x80x94B in the front sectional views. FIG. 7c, FIG. 8c and FIG. 9c are sectional views taken along line Cxe2x80x94C in each of the front sectional views, respectively.
At first, as shown in FIGS. 7a to 7c, the workpiece W formed with rack teeth at the second step is supported on the lower die set 320 including the left lower die 322, the central lower die 323 and the right lower die 321. In addition, a pair of side lower dies 324 are arranged at the both sides of the central lower die 323 and they support the side lower portion of the workpiece W. The upper surfaces of these dies have such a shape that the workpiece W passed through the second step is well fitted. In addition, the upper die 310 has, at its lower part, a semi-circular concave surface having a diameter that is approximately the same as a clearance at the legs of the workpiece W.
Then, when the upper die 310 descends, both legs 6 of the workpiece W are guided by the semi-circular concave surface and start to deform under application of bending force directed inwardly. At this time, although a high bending moment is applied near the roots of the legs 6, only a low bending moment is applied to the location near the extremity ends of the legs 6. Due to this fact, as shown in FIGS. 8b and 8c, deformation progresses only at the location near the roots of the legs 6 and the portions near the extremity ends of the legs 6 are scarcely deformed.
Due to this fact, as shown in FIGS. 9a to 9c, even if the upper die 310 descends down to the stroke end, a linear portion 7 undeformed into an arc shape remains near the extremity ends of the legs 6. As a result, since the sectional shape is not a true circle, the finished rack shaft becomes an inferior product. Further, since the butted end surfaces are not properly faced to each other, the end surfaces are inclined to form a V-shaped groove, so that they may become hindrance when a welding such as a laser welding and the like is performed.
It is required that a cylindrical shape without strain is formed by smoothly connecting the semi-circular bottom portion formed at the first step and the back arc portion formed at the third step, so that such a rack shaft as above is repaired by cutting or grinding operation or it is wasted as an improper product.
If it is assumed that the rack shaft having a deformed cylindrical part and the deformed semi-circular portion of the rack is attained, the grinding margin at the subsequent grinding step must have a large amount in order to correct this deformed sectional shape into a right circular shape. This process may become a problem that not only increasing manufacturing cost but also decreasing strength of the rack shaft as grinding margin increases, as a result, the rack shaft is damaged or its lifetime is shortened. If the thickness of the original plate member is made thicker in compliance with the grinding margin to prevent a reduction in strength, other portion of the completed hollow rack shaft becomes thicker than required, so that this may become a problem that the product may not satisfy the required weight reduction.
This invention aims at resolving these problems, and in this invention, a hollow rack shaft is manufactured through the first step, second step and third step, respectively. At the first step, a substantially rectangular plate workpiece is formed into a gutter-like shaped workpiece by press-forming. The gutter-like shaped workpiece has a bottom portion and a pair of leg-like side walls extending from the side edges of the bottom portion and substantially in parallel from each other. The bottom portion comprises a substantial flat bottom portion at a central portion in a longitudinal direction and semi-circular bottom portions at both outer sides. At the second step, a row of rack teeth is press-formed at the aforesaid flat bottom portion of the workpiece. At the third step, each of the pair of leg-like sidewalls of the workpiece formed at the second step is bent into a semi-circular shape to be butted to each other.
At the first step of this invention, further the first die set is used, wherein the first die set is provided with a surface such as complementary shape to each of the two semi-circular bottom portions in the gutter-like shape, and the surface such as the complementary shape is inclined to eliminate elastic deformation generated at the workpiece when the workpiece is removed from the first die set.
At the one second step in this invention, the second die set is used, the second die set is provided with a die surface such as complementary shape to a row of rack teeth formed at the flat bottom portion, the surface such as the complementary shape also has a shape for canceling elastic deformation of this workpiece when the workpiece is removed from the second die set.
At another second step of this invention, the second die set is used, the second die set is provided with a die surface such as complementary shape to a row of rack teeth formed at the flat bottom portion, a part of the die surface such as complementary shape is provided with recess for releasing the surplus material of the workpiece in the die clearance at the stroke end of the press-forming. The second die set can be divided into a plurality of segments at the location of recess. The recess is located at a position where a protrusion formed by the recess does not interfere with a pinion engaged with the rack.
The other second step of this invention comprises a preliminary forming step and a main forming step, wherein at the preliminary forming step, a row of teeth, having substantially the same pitch as the pitch of a row of rack teeth finally formed and having a smaller pressure angle than a pressure angle of the rack teeth finally formed, is formed. And at the main forming step, a row of teeth formed at the preliminary forming step is reformed into a row of rack teeth having required rack teeth shape. Further, at the main forming step, a part of the die surface is provided with a recess for releasing surplus material of the workpiece in the die gap, and it is possible to use the die set composed of a plurality of segments divided at the location of the aforesaid recess.
A third step in this invention is executed such that a mandrel, having at a part thereof a cylindrical outer surface corresponding to a hollow inner surface of the rack shaft, is inserted between the aforesaid pair of leg-like sidewalls. Further, this mandrel comprises at least two separable segments in such a way that it can be pulled out of the hollow rack shaft upon completion of the third step.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrates an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference made to the claims which follow by the description for determining the scope of the invention.